Spacelab 2 electron beam wave stimulation: Studies of important parameters

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The Spacelab 2 space shuttle mission included experiments on the production of waves in the ionospheric plasma by the injection of pulsed electron beams. The Spacelab 2 mission took place during July and August of 1985 and provided an extensive data set which continues to yield new results. The experimental results reported here were obtained with the combined use of the University of Iowa plasma diagnostics package (PDP) and the Stanford/Utah State vehicle charging and potential experiment (VCAP). To date most of the analysis has focused on the sequences performed during the release of the PDP as a free‐flying satellite. However, over 300 beam sequences were conducted with the PDP mounted in the orbiter's payload bay. The results of these experiments provide important new information which helps answer some of the outstanding questions which remain. The wave environments which exist in the orbiter payload bay and at several hundred meters away are compared both during ambient conditions and during electron beam injections. The dependence of beam‐generated wave characteristics on the duty cycle and the frequency of the pulsed beam are investigated. The duty cycle of the beam is found to be one of the most important parameters affecting the amplitude and the harmonic structure of the waves. Maximum amplitudes are produced by 50% duty cycles and amplitudes are proportional to instantaneous, rather than average, beam current. Harmonic structure also provides insight into the spatial integrity of the propagating beam. The dependence on pulsing frequency is found to show good agreement with theory for whistler mode waves produced through the Cherenkov resonance confirming previous results. Theoretical predictions for the dependence of wave amplitudes on parameters which could not be experimentally investigated are also provided. The results of both the experimental and theoretical studies have important practical consequences for experiments attempting to produce and detect propagating radiation using artificial electron beams.